XOM is designed to be easy to learn and easy to use. It works very straight-forwardly, and has a very shallow learning curve. Assuming you're already familiar with XML, you should be able to get up and running with XOM very quickly.

XOM is the only XML API that makes no compromises on correctness.
XOM only accepts namespace well-formed XML documents, and only allows you to create namespace well-formed XML documents. (In fact, it's a little stricter than that: it actually guarantees that all documents are round-trippable and
have well-defined XML infosets.) XOM manages your XML so you don't have to.
With XOM, you can focus on the unique value of your application, and trust XOM to get the XML right.

XOM is fairly unique in that it is a dual streaming/tree-based API. Individual nodes in the tree can be processed while the document is still being built. The enables XOM programs to operate almost as fast as the underlying parser can supply data. You don't need to wait for the document to be completely parsed before you can start working with it.

XOM is very memory efficient. If you read an entire document into memory, XOM uses as little memory as possible. More importantly, XOM allows you to filter documents as they're built so you don't have to build the parts of the tree you aren't interested in. For instance, you can skip building text nodes that only represent boundary white space, if such white space is not significant in your application. You can even process a document piece by piece and throw away each piece when you're done with it. XOM has been used to process documents that are gigabytes in size.

XOM includes built-in support for a number of XML technologies including
Namespaces in XML, XPath 1.0, XSLT 1.0,
XInclude, xml:id, xml:base, Canonical XML, and Exclusive Canonical XML.
XOM documents can be converted to and from SAX and DOM.

XOM is not complete unto itself. It depends on an underlying SAX
parser to read documents and feed the data into a tree structure.
While theoretically any SAX2 compliant parser should work, Xerces 2.6.1 and later
is the only one that I am fairly confident does work. Xerces 2.8.0
is included with the full distribution. This product includes
software developed by the Apache Software Foundation
(http://www.apache.org/). Piccolo 1.0.3, Crimson, GNU JAXP 1.0b1,
the Oracle XML Parser for Java 9.2.0.2.0D and 9.2.0.5.0, and Xerces
versions prior to 2.6.1 all have bugs that prevent them from doing
what XOM needs them to do. (Note to XML parser vendors: XOM's test
suite gives parsers a very thorough workout, and delves into many of
the more obscure parts of the XML spec that many parsers get wrong.
You could do a lot worse for testing than making sure all the XOM
unit tests pass when using your parser.)

Similarly XSLT support depends on a TrAX processor
(Xalan-J 2.6.0 is bundled). XInclude and
XML canonicalization, however, are native.

The current version of XOM is 1.2.10 and is backwards compatible with 1.2, 1.1 and 1.0.
You should not need to recompile any code to upgrade to 1.2.10.
XOM is believed to be quite stable and robust.
Future releases should be backwards compatible with the 1.0 API for the foreseeable future.

If you'd like to know more about XOM, I suggest starting with the tutorial.
XOM also includes a large collection of small sample programs that demonstrate various parts of the library.
If you're curious about why XOM is the way it is, or if you would like to suggest future directions for XOM,
you should read the design principles on which XOM is based.
If you have a question about XOM that is not answered in the API documentation
or the FAQ, you can ask it on the xom-interest
mailing list.
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If you don't get an answer, please subscribe and try again.)

Commercial licenses are available for companies that wish to bundle XOM with their products without subscribing to the LGPL. Contact Elliotte Rusty Harold if you're interested.